In the case where head == 0 on the circular buffer, there should be one
DMA buffer, not two.  The second zero-length buffer would break the
lpuart driver, transfer would never complete.
Signed-off-by: NAaron Brice <aaron.brice@datasoft.com>
Acked-by: NStefan Agner <stefan@agner.ch>
Tested-by: NStefan Agner <stefan@agner.ch>
Tested-by: NBhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>